backend: revert PR 201 because to make the test accurate

[djshow832]

What problem does this PR solve?

Issue Number: close None

Problem Summary:
#201 tried to make the test faster but there's a misunderstanding here:
I want to prove that the status of closeStatus is always statusNotifyClose and will never change to statusClosing or statusClosed, so I waited for 300ms.
If it's changed to require.Eventually, it will definitely return on the first try.

What is changed and how it works:
Revert the code.

Check List

Tests

• Unit test
• Integration test
• Manual test (add detailed scripts or steps below)
• No code

Notable changes

• Has configuration change
• Has HTTP API interfaces change (Don't forget to add the declarative for API)
• Has tiproxyctl change
• Other user behavior changes

Release note

Please refer to Release Notes Language Style Guide to write a quality release note.

None